Query optimization is important in relational database systems that deal with complex queries against large volumes of data. Unlike earlier navigational databases, a query on a relational database specifies what data is to be retrieved from the database but not how to retrieve it. Optimizing a query against a relational database is not as important in transaction-oriented databases where only a few rows are accessed either because the query is well specified by virtue of the application or because the query causes the data to be accessed using a highly selective index. In decision support and data mining applications, where the space of possible solutions is large and the penalty for selecting a bad query is high, optimizing a query to reduce overall resource utilization can provide orders of magnitude of overall performance improvement.
One existing query optimization technique is to rewrite the user-specified query. The query is transformed into a logically equivalent query that costs less, i.e., requires less time, to execute. The existing techniques for query transformation include syntactic and semantic techniques. Syntactic or algebraic transformations use the properties of the query operators and their mapping to rewrite the query. Some forms of magic set transformation, most forms of predicate push down, and transitive closures are techniques that fall under this category. Semantic query transformations use declarative structural constraints and the semantics of an application's specific knowledge, declared as part of the database, to rewrite the query. Semantic query transformation based rewrites are called semantic query optimization or SQO.
An SQO technique that bases a query rewrite on the current values stored in the database can raise issues when a concurrent transaction requests an insertion, deletion, or modification of those values. For example, an SQO can rewrite a query to reduce the numbers of rows accessed based on a known relationship between values in particular columns. If, however, the values are changed or a record is inserted with columns having values that do not meet the relationship, an inaccurate query result could be produced.